The relaxation time of the PR effect rises sharply when the device is cooled down to 1.8 [1]}, [2]}. As the cavity being illuminated at cryogenic temperature, the space-charge field induced by the PR effect accumulates and leads to long-lived modulation of resonances in the cavity.
A continuous blue shift of all resonances is observed when we repeatedly scan the laser from 1500 to 1600 within the transmission window of the grating couplers with an input power of 5 into to fridge (-6.5dBm on the input waveguide).
The blue shift of resonance reaches a saturation after around thirty minutes of scanning. The normalized spectrum before scanning and after saturation is shown in Fig. REF . The high-extinction mode group we study in this letter is the fundamental TE00 mode of the ring resonator with radius of 80, which has an free spectral range (FSR) of  2.1 around 1550 and average loaded Q of \(6\times 10^5\) . A universal blue shift of 0.76 is observed in the TE00 mode group. The saturation behaviour can be explained by the limited defect density available for charge excitation and trapping process in LN crystal [2]}, [4]}. The average loaded Q of TE00 mode group remains approximately unchanged before and after the blue shift. We also record the time-dependent relaxation of resonances at 1.8. The TE00 mode group experience a relaxation of 0.21 after four days, and a characteristic time constant of more than ten days is obtained from the exponential fitting of the recorded shift trace.
